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Please bear with me...

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Please bear with me...

Post #1 by 1986F150six » Thu Feb 20, 2014 9:58 am


I have a question which can best be answered by you "small block six" gurus. It has to do with specific carburetion applications.

I drive a 1986 F150 with 4.9L engine and 4 speed manual OD. It has been converted to Duraspark ignition and the feedback carburetor was replaced with a Carter YF from a 1970 F350 [no emmissions] and the distributor recurved to better deal with no EGR. Overall, this system works quite well and returns respectable fuel mileage. I drive very conservatively and monitor/record every tank. The engine rarely sees beyond 2200-2500 rpms and often is in the 1500-2000 rpm range.

I have two ideas I would like to play with and would appreciate any and all feedback!

Idea #1 is mount a smaller carburetor to see what difference is noted in throttle response and mileage.

Idea #2 is to modify a stock log intake and mount two of the smaller carburetors in an attempt to have better fuel distribution.

I would like to stay with OEM type carburetors to make it easier regarding air breathers, heated air intake, parts, etc.

I looked on Rockauto just for reference and found that on the oldest models, for which they have a listing, there are three different carburetors for the 170, 200 and 250. The one for the 250 shows a side fuel bowl.

I have read that "early" small six carburetors had something to do with the spark timing [LOM?] and certainly want to avoid that. I am currently running ported vacuum to the distributor.

The smaller the base [foot print] the easier it will be to modify the log intake manifold. Can someone provide base dimensions?

Finally, what are the approximate CFM ratings of each of the carburetors listed above and which might you recommend for best fuel mileage?

Sorry for the long-winded post and thank you!

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Re: Please bear with me...

Post #2 by JackFish » Thu Feb 20, 2014 11:31 am


Sorry, couldn't resist.... :lol:
Off the top of my head, a Carter RBS might fit the bill.
It's got the most CFM of the carbs in it's class.
And there were two YF's I think, the one that is larger are found on trucks.
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Re: Please bear with me...

Post #3 by 1986F150six » Thu Feb 20, 2014 11:38 am

Cute! Thanks for your input, JackFish.

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Re: Please bear with me...

Post #4 by bubba22349 » Thu Feb 20, 2014 11:57 am

In my Opinion you can get slightly better economy with an Autolite 1V carb they were used on the early 240 engines. Its tough to find a good one where the throttle shaft is not loose but if your handy you could install some bushings and a new shaft. Another early carb that might work is a Holley 1904. As far as the LOD part of the carb you can plug that off or convert it to work with a vacuum advance. On your idea to use two carbs I think you have a number of ways you could go I have been thinking about the same thing for awhile the problem is there are no intakes made to do it so you would need to cut and weld a couple of stock ones together It probably can be done fairly easy check out the 3 x 1V intakes (look at FTF posts) that have been made. As for carbs again the Holley's, Autolite's, Carter's and if I can find some intakes to cut up or might just weld up a complete intake from scratch to try two of the Pinto progressive 2V carbs (5200), there are probably many more carb combo's that would also work. Good luck :nod:
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Re: Please bear with me...

Post #5 by rbohm » Thu Feb 20, 2014 12:50 pm

this belongs in the big six forum.
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Re: Please bear with me...

Post #6 by blmhawkeye » Thu Feb 20, 2014 3:08 pm

Dual plane offy with a 390 and the secondaries disabled could probably do the trick. ur gonna be hard pressed getting the 1v autolite to work for ya as they have alot of years on them. the one I have does nothing but leak fuel everywhere because it has such a smal surface area for the fuel bowl gasket.

What is your diff gear ratio? iff its 3.5 or higher look into lowering to get some better fuel mileage.

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Re: Please bear with me...

Post #7 by 1986F150six » Thu Feb 20, 2014 3:29 pm

Thank you all for your responses. The reason I posted in the "small six" forum was to draw from their experience with the smaller carburetors.

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